nvdimm: Fix error path mis-free

(diff applied manually due to refactoring)

Regression introduced in r343629 when malloc result was renamed from spa to
spa_mapping and the 'spa' name was instead used to iterate a table, but the
free() target was not updated.

Reviewed by: kib, scottph
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D21871

(cherry picked from commit 31f1c8fc84c850d73e74781126e12efceb50032b)